Method of coating a device, particularly a heat exchanger tube

ABSTRACT

A method of making a heat exchanger tube having higher corrosion resistance is provided. The heat exchanger tube includes an Al alloy extruded tube, and a flux layer containing a Si powder and a Zn-containing flux formed on the external surface of the Al alloy extruded tube, wherein an amount of the Si powder applied to the Al alloy extruded tube is not less than 1 g/m 2  and not more than 5 g/m 2 , and an amount of the Zn-containing flux applied to the Al alloy extruded tube is not less than 5 g/m 2  and not more than 20 g/m 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat exchanger tube and, moreparticularly, relates to a heat exchanger tube having high corrosionresistance.

Priority is claimed on Japanese Patent Application No. 2003-128170,filed May 6, 2003, the content of which is incorporated herein byreference.

2. Description of the Related Art

As shown in FIG. 2, a heat exchanger generally comprises a pair of rightand left pipe bodies called header pipes 5, a multitude of tubes I madeof an aluminum alloy installed in parallel at intervals from each otherbetween the header pipes 5, and fins 6 installed between the tubes 1, 1.The inner space of each of the tubes 1 and the inner space of the headerpipes 5 communicate with each other, so as to circulate a medium throughthe inner space of the header pipes 5 and the inner space of each of thetubes 1, thereby achieving efficient heat exchange via the fins 6.

It is known to constitute the tubes I of the heat exchanger from heatexchanger tubes 11 made by coating the surface of an Al alloy extrudedtube 3, that has flattened cross section and a plurality of holes 4 forpassing the medium as shown in perspective view of FIG. 1, with a fluxcontaining a brazing material powder so as to form a flux layer 2. It isalso known to make the Al alloy extruded tube 3 from material (JIS1050)that has high workability for extrusion forming process, and to use a Sipowder, an Al—Si alloy powder or an Al—Si—Zn alloy powder as the brazingmaterial contained in the flux layer 2.

A heat exchanger is manufactured using the conventional heat exchangertube 11 described above in a process such as: the heat exchanger tubes11 are installed at right angles to the header pipes 5 that are disposedin parallel at a distance from each other, ends of the heat exchangertubes 11 are inserted into openings (not shown) that are provided in theside face of the header pipe 5, the fins 6 having corrugated shape areassembled between the heat exchanger tubes 11, and the assembly isheated in a heating furnace so that the header pipes 5 and the tubes 1are fastened to each other by brazing with the brazing material providedon the heat exchanger tube 11 while the fins 6 of corrugated shape arefastened between the tubes 1, 1 by brazing.

Wall thickness of the tube 1 that constitutes the heat exchanger is madesmaller than that of the header pipe 5 in order to achieve highefficiency of heat exchange. As a result, in the case in which the tubeand the header pipe are corroded at comparable rates, it is likely thata penetrating hole will be formed by corrosion first in the tube therebyallowing the medium to leak therethrough. Thus, it has been a majorconcern in the heat exchanger to prevent corrosion of the tubes.

In order to improve the corrosion resistance of the heat exchanger tube11, a sacrificial anode layer containing Zn as a major component isformed on the surface of the tubes in the conventional heat exchangers.As the process to form the sacrificial anode layer, such processes areknown as thermal spraying of Zn and coating with a Zn-containing flux.Japanese Patent Application Unexamined Publication No. 7-227695discloses an example that employs Zn-containing flux.

However, when forming the sacrificial anode layer by thermal spraying,it is difficult to precisely control the amount of metal applied bythermal spraying, thus leading to such a problem that the sacrificialanode layer cannot be formed uniformly on the tube surface, and thecorrosion resistance of the tube cannot be improved.

When the Zn-containing flux described in Japanese Patent ApplicationUnexamined Publication No. 7-227695 as mentioned above is used, it maybe believed that corrosion resistance of the tube can be improved sincethe flux and Zn are supplied simultaneously onto the tube surface. Inactuality, however, it is difficult to achieve a stable coatingcondition with ordinary coating methods such as immersion coating androll coating, and therefore it has been difficult to uniformly apply theZn-containing flux. As a result, Zn distribution in the sacrificialanode layer becomes uneven, thus leading to insufficient corrosionresistance of the tubes with preferential corrosion occurring in aportion that has higher concentration of Zn.

SUMMARY OF THE INVENTION

The present invention, which has been completed in view of thebackground described above, has an object of providing a heat exchangertube that has higher corrosion resistance.

In order to achieve the object described above, the present inventionemploys the following constitution.

The heat exchanger tube of the present invention comprises an Al alloyextruded tube, and a flux layer containing a Si powder and aZn-containing flux formed on the external surface of the Al alloyextruded tube, wherein an amount of the Si powder applied to the Alalloy extruded tube is not less than 1 g/m² and not more than 5 g/m²,and an amount of the Zn-containing flux applied to the Al alloy extrudedtube is not less than 5 g/m² and not more than 20 g/m².

The Zn-containing flux preferably contains at least one Zn compoundselected from ZnF₂, ZnCl₂ and KZnF₃.

When such a heat exchanger tube is used, since a mixture of the Sipowder and the Zn-containing flux is applied, the Si powder melts andturns into a brazing liquid during a brazing process, and Zn containedin the flux is diffused uniformly in the brazing liquid and isdistributed uniformly over the tube surface. Since the diffusionvelocity of Zn in a liquid phase such as the brazing liquid issignificantly higher than the diffusion velocity in the solid phase, Znconcentration in the tube surface becomes substantially uniform, thusmaking it possible to form a uniform sacrificial anode layer and improvethe corrosion resistance of the heat exchanger tube.

It is preferable for maximum particle size of the Si powder to be 30 μmor less. Maximum particle size larger than 30 μm results in an increasein the erosion depth of the tube and is therefore not desirable. Whenmaximum particle size of the Si powder is less than 0.1 μm, Si particlesclump, and the erosion depth of the tube increases also in this case.Therefore, the maximum particle size is preferably not less than 0.1 μm.

The Al alloy extruded tube is preferably made of an Al alloy containingSi and Mn, with the balance being Al and inevitable impurities, while aSi content is 0.5% by weight or more and 1.0% by weight or less, and aMn content is 0.05% by weight or more and 1.2% by weight or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger tube of the prior art.

FIG. 2 is a perspective view of a heat exchanger of the prior art.

DETAILED DESCRIPTION OF THE INVENTION

Next, preferred embodiments of the present invention will be describedin detail.

The heat exchanger tube of the present invention is made by forming theexternal surface of an Al alloy extruded tube with a flux layercontaining a Si powder and a Zn-containing flux.

The Al alloy extruded tube which constitutes the heat exchanger tube ismade of an Al alloy containing Si and Mn, with the balance being Al andinevitable impurities, where a Si content is 0.5% by weight or more and1.0% by weight or less, and a Mn content is 0.05% by weight or more and1.2% by weight or less.

The reason for restricting the composition of the Al alloy extruded tubewill be described below. Si has an effect in that a large amount of Siforms a solid solution in the Al alloy extruded tube, thus resulting innoble potential of the Al alloy extruded tube, and causes preferentialcorrosion to occur in the header pipes and the fins that are brazed withthe tubes when assembling the heat exchanger, thereby suppressing deeppitting corrosion from occurring in the Al alloy extruded tube, whileimproving the brazing characteristic and forming good joint thereby toimprove the strength after brazing. Si content of less than 0.5% cannotachieve the desired effect, and is therefore not desirable. Si contenthigher than 1.0%, on the other hand, lowers the melting point of thealloy resulting in excessive melting during brazing and poor extrusionforming characteristic, and is not desirable. Therefore, Siconcentration in the Al alloy extruded tube is set in a range from 0.5to 1.0%. More preferable range of Si concentration is from 0.6% to 0.8%.

Mn has the effect of turning the Al alloy extruded tube to noblepotential and, because of less likelihood of diffusing in the brazingmaterial, allows higher potential difference with the fin or the headerpipe so as to make the corrosion preventing effect of the fin or theheader pipe more effective, thereby improving the external corrosionresistance and the strength after brazing. Mn content of less than 0.05%cannot achieve sufficient effect of turning the Al alloy extruded tubeto noble potential, and is therefore not desirable. Mn content higherthan 1.2%, on the other hand, results in poor extrusion formingcharacteristic, and is not desirable.

Therefore, Mn concentration in the Al alloy extruded tube is set in arange from 0.05 to 1.2%.

The flux layer formed on the tube surface contains the Zn-containingflux and the Si powder, so that a molten brazing material layer isformed over the entire surface of the tube after brazing. Since thebrazing material layer contains Zn uniformly distributed therein, thebrazing material layer has similar effect as that of the sacrificialanode layer so that the brazing material layer is subject topreferential planar corrosion. Therefore deep pitting corrosion can besuppressed and corrosion resistance can be improved.

The amount of a Si powder applied to the heat exchanger tube ispreferably not less than 1 g/m² and not more than 5 g/m². When theamount is less than 1 g/m², sufficient brazing strength cannot beachieved because of insufficient amount of the brazing material, andsufficient diffusion of Zn cannot be achieved. When the amount is morethan 5 g/m², Si concentration in the tube surface increases and the rateof corrosion increases, and is therefore not desirable.

The flux layer contains at least the Zn-containing flux. In addition tothe Zn-containing flux, a flux which does not contain Zn may also becontained.

The Zn-containing flux preferably contains at least one Zn compoundselected from ZnF₂, ZnCl₂ and KZnF₃. The flux which does not contain Znpreferably contains at least one fluoride such as LiF, KF, CaF₂, AlF₃ orSiF₄ or a complex compound of the fluoride such as KAlF₄ or KAlF₃.

As the Zn-containing flux is contained in the flux layer of the heatexchanger tube, a Zn-diffused layer (brazing material layer) is formedon the tube surface after brazing, so that the Zn-diffused layerfunctions as a sacrificial anode layer, thereby improving theanti-corrosion effect of the tube.

Also, because a mixture of the Si powder and the Zn-containing flux isapplied, the Si powder melts and turns into a brazing liquid during abrazing process, Zn contained in the flux is diffused uniformly in thebrazing liquid and is distributed uniformly over the tube surface. Sincediffusion velocity of Zn in a liquid phase such as the brazing liquid issignificantly faster than the diffusion velocity in a solid phase, Znconcentration in the tube surface becomes substantially uniform, thusmaking it possible to form a uniform Zn-diffused layer and improve thecorrosion resistance of the heat exchanger tube.

The amount of the Zn-containing flux applied to the heat exchanger tubeis not less than 5 g/m² and not more than 20 g/m². An amount of lessthan 5 g/m² results in insufficient formation of a Zn-diffused layerthat does not have sufficient anti-corrosion effect, and is thereforenot desirable. An amount of more than 20 g/m² causes excessive Zn to beconcentrated in a fillet that is the joint of the fin with othercomponents which results in higher rate of corrosion in the joint, andis therefore not desirable.

The heat exchanger can be constituted by brazing the heat exchangerheader pipes and the fins to the heat exchanger tube described above.

That is, the heat exchanger is constituted from the heat exchanger tubeof the present invention, the heat exchanger header pipes and the finsthat are joined with each other. Similarly to the heat exchangerdescribed in conjunction with the prior art, the heat exchangercomprises a pair of right and left pipe bodies called “heat exchangerheader pipes”, a plurality of heat exchanger tubes installed in parallelat intervals from each other between the heat exchanger header pipes,and fins installed between the heat exchanger tubes. The inner space ofthe heat exchanger tube and the inner space of the heat exchanger headerpipe are communicated with each other, so as to circulate a mediumthrough the inner space of the heat exchanger header pipe and the innerspace of the heat exchanger tube, thereby to achieve efficient heatexchange via the fins.

EXAMPLES

Al alloy extruded tubes having 10 cooling medium passing holes and crosssection measuring 20 mm in width, 2 mm in height and wall thickness of0.20 mm were produced, by extrusion forming of billets made of an Alalloy containing 0.7% by weight of Si and 0.5% by weight of Mn.

Then a flux mixture was prepared by mixing the Zn-containing flux to Sipowder. The flux mixture was applied by spraying onto the outer surfaceof the Al alloy extruded tube that was produced in advance, therebyforming a flux layer. The amounts of the Si powder and the flux mixtureapplied to the Al alloy extruded. tube are shown in Table 1. Thus theheat exchanger tubes of Examples 1 to 6 and Comparative Examples 1 to 4were produced.

Then fins made of cladding material (JIS3003 or JIS3003/JIS4045) wereassembled on the heat exchanger tubes of Examples 1 to 6 and ComparativeExamples 1 to 4, and the assemblies were kept at 600°C. in a nitrogenatmosphere for three minutes so as to carry out brazing. The tubes withthe fins brazed thereon were subjected to corrosion tests (SWAAT, 20days) to measure the maximum corrosion depth of the tubes. The testresults are shown in Table 1. TABLE 1 Si powder Flux Amount of MaximumAmount of Maximum coating particle coating corrosion (g/m²) size (μm)Composition (g/m²) Fin depth (μm) Remark Example 1 1 10 KZnF₃ 5 JIS300375 — Example 2 3 10 KZnF₃ 10 JIS3003 70 — Example 3 5 10 KZnF₃ 15JIS3003 80 — Example 4 5 10 KZnF₃ 20 JIS3003 75 — Example 5 3 10 ZnCl₂ +KAlF₄ 10 + 10 JIS3003 95 — Example 6 3 35 KZnF₃ 10 JIS3003 80 somewhatdeep erosion Comparative 3 10 KAlF₄ 10 JIS3003 350 — Example 1Comparative 3 10 KZnF₃ + KAlF₄  2 + 10 JIS3003 300 — Example 2Comparative — — ZnF₂ 10 JIS3003/ 175 — Example 3 JIS4045 Comparative — —KZnF₃ 20 JIS3003/ 200 — Example 4 JIS4045

As shown in Table 1, maximum corrosion depth was less than 100 μm in anyof the finned tubes of Examples 1 to 6, indicating that corrosion of thetubes was suppressed. Example 6 showed a little deeper erosion becauseof larger maximum particle size of the Si powder.

Extent of corrosion was larger in the comparative examples, presumablybecause Zn was not added to the flux in Comparative Example 1, smalleramount (2 g/m²) of the Zn-containing flux (KZnF₃) was added inComparative Example 2, and Zn was distributed unevenly since Si powderwas not added in the comparative examples 3 and 4.

As described in detail above, in the heat exchanger tube of the presentinvention, since the mixture of the Si powder and the Zn-containing fluxis applied, the Si powder melts and turns into a brazing liquid during abrazing process, while Zn contained in the flux is diffused uniformly inthe brazing liquid and is distributed uniformly over the tube surface.Since diffusion velocity of Zn in a liquid phase such as the brazingliquid is significantly higher than diffusion velocity in a solid phase,Zn concentration in the tube surface becomes substantially uniform, thusmaking it possible to form a uniform sacrificial anode layer and improvethe corrosion resistance of the heat exchanger tube.

Since the amount of the Zn-containing flux is in a range not less than 5g/m² and not more than 20 g/m², Zn can be distributed uniformly over thetube surface.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1-4. (canceled)
 5. A method of producing a heat exchanger comprising:applying Si powder to an external surface of an Al alloy extruded tubewith an extrusion profile including a plurality of internal passages ina quantity of at least 1 g/m² and not more than 5 g/m², and furtherapplying an amount of Zn-containing flux mixed with the Si powder, in aquantity of at least 5 g/m² and not more than 20 g/m²; and heating theAl alloy extruded tube to a temperature sufficient to fuse the Si powderso as to braze the Al alloy extruded tube to header pipes and fins ofthe heat exchanger.
 6. The method according to claim 5, wherein theZn-containing flux contains at least one Zn compound selected from ZnF₂,ZnCl₂, and KZnF3.
 7. The method according to claim 5, wherein themaximum particle size of the Si powder is 30 μm or smaller.
 8. Themethod according to claim 5, wherein the Al alloy extruded tubecontains, by weight, between about 0.5% and about 1.0% Si, and betweenabout 0.05% and about 1.2% Mn, with a balance being substantially Al. 9.The method according to claim 5, wherein the Zn-containing flux containsat least one Zn compound selected from the group consisting of ZnF₂ andKZnF₃.
 10. The method according to claim 5, wherein the applying ofZn-containing flux mixed with the Si powder comprises spraying.